The invention relates to a new ternary rare earth sulfur compound. More specifically the invention relates to a new lanthanum rare earth sulfur compound which is stable at high temperatures and which has good high temperature, n-type theremoelectric properties.
Lanthanum sulfur compounds of the formula La.sub.3 S.sub.4 are known to possess good thermoelectric properties. However, when these compounds are subjected to temperature above about 800.degree. C., phase changes occur within the material which, when it is subsequently cooled, result in the formation of small cracks within the material. This cracking reduces the electrical conductivity of the material, and consequently reduces or destroys the thermoelectric properties. Efforts to stabilize these compounds to prevent or reduce the phase changes have included the addition of small amounts of some of the alkaline earth elements as described in U.S. patent application Ser. No. 470,114 filed Feb. 28, 1983. As described therein, from about 0.1 to about 5 weight percent of calcium, barium, or strontium is added to the lanthanum sulfide to stabilize the compound in the preferred cubic phase.